Is there any particular reason you need to know the trim of the truck? Just pick the trim level you like and fix it up with parts that match.
My '75 has Silverado trim and I have the original SPID to get colors and other details. However, I have deviated from original in several areas. For example, my hood and front fenders are reproduction items that don't have quite the right shape for a '75, and my entire bed is one I bought in the junkyard off an 80's truck. I'm using 16" tires and wheels instead of the original 16 1/2" split rims because 16's are cheaper and easier to find. I'm also using newer design mirrors and the grill is from a late 70's truck. If someone really knows squarebody trucks they would very quickly see those mismatched details on my '75 , but the only comment I ever get is "nice truck".
The first picture is what my truck looked like when I got it from my family farm. The 2nd picture is what it looked like when my son started driving it in high school. The third picture is from last year, which is about 17 years after the original body work and paint job. The truck is still working hard.